In a known system for cleaning the exhaust gas of an internal combustion engine, a sensor adapted to provide an electric signal is utilized for detecting the opening of a throttle valve of the engine. In the prior art, a so-called vacuum switch has been used to carry out the function of the above-mentioned sensor. This vacuum switch has a diaphragm which is mechanically connected to a switch mechanism and which forms a vacuum chamber opened to the intake line of the engine. The diaphragm operates to cause the switch mechanism to provide an electric signal when a vacuum level in the intake system of the engine reaches a predetermined value for detecting a predetermined position of the throttle valve.
However, this type of switch has a drawback in that the position of the throttle valve cannot be accurately detected, because the level of the vacuum in the intake passageway does not correspond precisely to the position of the throttle valve. Due to this drawback, an ideal operation of the exhaust gas cleaning apparatus cannot be obtained. Therefore, a large amount of toxic components is exhausted into the atmosphere without being cleaned.